Well, sounds like it might indeed be refrigerant.

Unfortunately, I have not found a car yet that allows you to remove the evaporator without cracking open the A/C system.

Leaks near the Thermostatic Expansion Valve (TXV) are rare, unless the vehicle is known for it. It's not that I doubt your diagnosis, simply that verifying the problem by removing the evaporator is painful if you are wrong, and painful if you are right, so I'm trying to see if there is a way to confirm this without removal of the evaporator.

Here's what I would do next:

First, check the sight glass while the system is running. If you see bubbles persisting while the compressor is on, then you are low on refrigerant.

I would add some dye to the system. You can buy cans of dye (with a small refrigerant charge) and add it the LOW SIDE of the system. If the low connection is at the compressor, care must be taken to avoid slugging the compressor; use a bowl of boiling hot water to dunk the can into in order to help push it in as a gas. (Use the can right side up for gas, upside down for liquid). It's probably safe to tip the can upside down for just an instant at first (while the compressor is running) to shoot just a little liquid into the system. Then, let the hot water heat and push out the rest as gas. The goal is to get dye into the system; refrigerant is just there as a propellant.

Letting the system completely decompress out of refrigerant is a bad idea unless you are prepared to replace the receiver/dryer, evacuate the system completely, and refill it with new refrigerant. The dessicant in the receiver dryer absorbs moisture and when it's exposed to air, it gets used up, and the small amount of moisture in the air causes corrosion inside the system.

I'm not saying don't service your own A/C; I recommend a new R/D, full evacuation (appropriate vacuum pump for 30 minutes) and a full recharge shortly thereafter.

Adding the dye to the system will allow you to diagnose a potential leak under the dash without taking it apart. The dye will make its way to the leak, and will eventually start to wash out the drain tube with the condensation. Even if it doesn't get this far, you should be able to see something with a small mirror on a stick and a flashlight with the glovebox removed.

If you dead set on removing the evaporator, you should drain the system at the lowside service valve (after the system has cooled so that it is ambient on both sides!) You may need a special valve adapter for the new style valves. Stand back, if the stuff comes out as a liquid, you can suffer severe frostbite on the skin or eyes. Also, whatever it blows onto may be damaged by the extreme cold (paint?). After it stops hissing, find the low and high side connectors where they enter the firewall. Disconnect them. They will likely pull through into the cabin.

Have at the evaporator from the inside. The evaporator may slide out, may be in an enclosure that is held in with screws or bolts, but it should come out of there without a full dashectomy.

Nate